The brain’s metabolic demands and fuel flexibility.
What the evidence says
- Brain accounts for ~2% of body weight but consumes ~20% of glucose-derived energy[1]
- Brain consumes approximately 120-130g glucose per day under normal conditions[1]
- Brain glucose consumption is continuous, supporting signaling, synaptic activity, and cellular maintenance[1]
- Neurons are largely intolerant of inadequate energy supply — the brain requires glucose not only for energy but also for biosynthetic pathways (amino acid synthesis, oxidative stress management) that ketones cannot replace[1]
- Glycogen stored in astrocytes is the brain’s only energy reserve; even very low rates of glycogenolysis (a few percent of normal glucose utilisation) can prolong neuronal function during hypoglycemia by sparing glucose for neurons[1]
- In prolonged fasting, ketones replace glucose as the brain’s predominant fuel — the widely quoted “60–70% of the brain’s energy” is a derivation from that paper’s arteriovenous and oxygen-equivalent figures, not a percentage the paper states, and its subjects were three obese patients at 5–6 weeks of therapeutic starvation[2]
- The brain adapts efficiently to ketones as alternative fuel[2],[3]
- D-β-hydroxybutyrate yields 31% more energy per carbon unit than pyruvate, explaining the energetic advantage of ketones as brain fuel[4]
- On the review’s account, the metabolic switch from glucose to ketones triggers adaptive brain responses[5]
- The review reports that ketones upregulate BDNF, supporting synaptic plasticity and cognitive enhancement — mechanism from preclinical work, not a measured human outcome[5]
- Also on the review’s account, and also preclinical: BHB induces BDNF expression, and the review says it may do so by inhibiting histone deacetylases (HDACs) that repress it (may restored 2026-09-10) — an epigenetic mechanism, not a measured human outcome[5]